The present invention is a layered electrophotographic photoconductor, i.e., a photoconductor having a metal ground plane member on which a charge generation layer and a charge transport layer are coated, generally in that order. Although these layers are generally separate from each other, they may be combined into a single layer which provides both charge generation and charge transport functions. Such a photoconductor may optionally include a barrier layer located between the metal ground plane member and the charge generation layer, an adhesion-promoting layer located between the barrier (or ground plane member) and the charge generation layer, and/or an overcoat layer on the top surface of the charge transport layer.
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of this surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners containing pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the photoconductor surface, either exposed or unexposed to light, depending upon the relative electrostatic charge on the photoconductor surface and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor surface, pulling the toner from that surface onto the paper or the transfer medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls melts and fixes the toner on the paper, subsequent to direct transfer or indirect transfer when an intermediate transfer medium is used, producing the printed image.
The electrostatic printing process, therefore, comprises an on-going series of steps in which the photoconductor surface is charged and discharged as the printing takes place. It is important to keep the charge voltage on the surface of the photoconductor relatively constant as different pages are printed to make sure that the quality of the images produced is uniform (cycling stability). If the charge/discharge voltage is changed significantly each time the drum is cycled, i.e., if there is fatigue or other significant change in the photoconductor surface, the quality of the pages printed will not be uniform and, as a result, will not be satisfactory.
As electrophotography matures, increasingly demanding applications are envisioned for it. For example, printers that produce an increased number of prints per minute are always being developed. In order to produce more prints per minute, such printers operate at higher process speeds. If laser output power remains fixed, then the higher process speed means that less laser energy per square centimeter will be available to discharge the photoconductor, hence higher sensitivity is required of the photoconductor to get high quality prints. Similarly, color printers that use a number of photoreceptors in a serial arrangement typically have low output speeds because the electrophotographic process must be repeated on each drum. In order to provide color output at acceptable speeds, process speeds must be increased and, again, the same increased photosensitivity is required. Further, color devices that use a number of photoreceptors in a serial arrangement must ensure that photoreceptor fatigue is minimal. Drums, representing the different colors to be printed, are electrically "written" or cycled to different degrees, depending upon the demand for each specific color in the final print. For example, the drum used for printing black would most likely be electronically cycled much more frequently than the drum used for printing magenta. In order to ensure faithful color reproduction over the useful life of the photoconductor, the drums cannot fatigue at different rates. This is best achieved by minimizing photoconductor fatigue.
It is relatively easy to improve either sensitivity or fatigue, but such beneficial modification of one parameter usually results in a worsening of the other property. For example, increased sensitivity can be obtained by simply adding more charge generating material to the photoreceptor. Unfortunately, this approach also leads to an increase in photoconductor fatigue and dark decay. Because it is relatively difficult to simultaneously improve both sensitivity and fatigue, ways to achieve such simultaneous improvement of photoreceptors are of value and are constantly being sought.
The present invention is based on the unexpected finding that the incorporation of simple quinones into either the charge generation layer comprising a phthalocyanine charge generation molecule, or the charge transport layer comprising an amine charge transport molecule, provides simultaneous improvement in both sensitivity and fatigue of a photoconductor. In addition, the photoconductor exhibits higher charge voltage, lower residual voltage and lower dark decay when compared with similar photoconductors which do not include the quinone component.
The use of quinones as a class of materials in laminated photoreceptors is not new. Large, polycyclic quinones, long recognized as dyes or pigments, have been used in the colorant industry for centuries. Hence, their use as light-absorbing charge generating molecules has been widely explored and documented. See, for example, U.S. Pat. No. 5,677,097, Nukada, et al., issued Oct. 14, 1997; U.S. Pat. No. 5,190,839, Fujimaki, et al., issued Mar. 2, 1993; U.S. Pat. No. 5,075,189, Ichino, et al., issued Dec. 24, 1991; and U.S. Pat. No. 3,877,935, Regensburger, et al., issued Apr. 15, 1975. When used as a charge-generating molecule, the quinone actually absorbs actinic radiation and begins the charge separation which is central to the electrophotographic process. In contrast to this, the present invention uses simple quinones, rather than large polycyclic quinone dyes or pigments, and the molecules do not absorb the actinic radiation or initiate the charge generation process.
Quinones have also been used in the charge transport layer of laminated photoreceptors as charge transport molecules in systems involving electron transport via radical anions through the charge transport layer. See, for example, Yamaguchi, Y. et al., Chem. Mater. 3: 709-714 (1991); European Published Patent Application 426 445 A2, Yokoyarna, et al., published May 8, 1991; European Published Patent Application 699 962 A1, Nogami, S., et al., filed Mar. 6, 1996; and European Published Patent Application 506 387 A2, Tanaka, et al., filed Sep. 30, 1992. In the present invention, the quinones are not used at levels where they can transport charge through the charge transport layer.
U.S. Pat. No. 5,707,766, Nogami, et al, issued Jan. 13, 1998, describes an electrophotographic member which exhibits stable electrical characteristics during repeated use. The electrophotographic member incorporates mixtures of hydrobenzoic acid compounds and quinone compounds in the charge transport layer. This patent exemplifies only DEH as the charge transport agent, a material which is not operable in the present invention.
U.S. Pat. No. 5,134,050, Eto, et al., issued Jul. 28, 1992, describes a photoreceptor having a light sensitive layer which includes a specific polycyclic quinone compound (containing at least six rings), a specific bisazo pigment, and a specific stilbene as a charge transport material. These photoreceptors are taught to be highly sensitive and capable of accurately reproducing red images. The quinones utilized in this invention are complex and are not the simple quinones used in the present invention.
U.S. Pat. No. 5,449,580, Nakamori, et al., issued Sep. 12, 1995, describes a photosensitive material used for electrophotography which contains a specific diphenoquinone as an electron-transporting agent. The diphenoquinone component must have at least one aryl substituent. The material is utilized as the electron transport material and, therefore, is utilized at a relatively high level.
Yamaguchi, et al., Chem. Mater. 3: 709-714 (1991), describes unsymmetrically substituted diphenoquinones at high loading as effective electron transport compounds for use in photoconductors. 3,5-dimethyl-3', 5'-di-t-butyl-4, 4' diphenoquinone is specifically disclosed.
EPO Published Patent Application 426 445, Yokoyama, et al., published May 8, 1991, describes a photosensitive material for use in electrophotography which comprises an organic polysilane as the charge transport substance and a number of other materials, one of which is a diphenoquinone derivative. The material is said to maintain charging stability on repeated uses and is also said to control fatigue. The disclosed electrophotographic members do not utilize the amine charge transport materials required in the present invention, but instead require the use of a polysilane charge transport material.